This application is based on key, recent findings highlighting new functions for RNA editing and RNA binding proteins in intestinal lipid metabolism, stem cell biology and tumorigenesis. First, our demonstration of unsuspected new functions and novel targets of Apobec-1 dependent C-to-U RNA editing leads us to examine how target selection and mechanisms of C-to-U RNA editing regulating a key gene (apoB) required for intestinal lipid mobilization have adapted to mediate RNA editing of other physiological targets. We will also examine targets of aberrant, Apobec-1 dependent, C-to-U RNA editing which are relevant for intestinal tumorigenesis in both mouse models and in humans. Secondly, we will also examine the mechanisms and functional impact of HuR dependent modulation of pathways involved in intestinal epithelial growth and tumorigenesis. The BACKGROUND to this proposal is based on (1) the identification of new targets for Apobec-1 dependent C-to-U RNA editing with functional impact on gene expression and (2) our findings of a growth phenotype in conditional intestinal HuR knockout mice (HuR-IKO) with protection against polyposis in compound, transgenic Apc Min-HuR IKO mice. Our OVERARCHING HYPOTHESES are that (1) Apobec-1 mediated C-to-U RNA editing regulates target gene expression, including and beyond apoB, in a species-specific manner. (2) Aberrant Apobec-1 mediated C-to-U RNA editing modulates targets and pathways that in turn regulate tumorigenesis. (3) HuR modulates intestinal growth and tumorigenesis via LGR5 dependent pathways. Based on our findings demonstrating tissue-specific physiological C-to-U RNA editing in a range of novel RNA targets, studies in AIM 1A will examine the mechanisms, players and functional impact of physiological C-to-U editing on human and murine apoB RNAs and two novel physiological targets. Based on our findings in mouse models and in human colon cancer cell lines, studies in AIM 1B will examine aberrant Apobec-1 mediated C-to-U RNA editing of two exonic targets in Apc Min models, enteroids, colon cancer cell lines and human CRC. Finally, based on our findings of defective intestinal growth and a tumor protection phenotype with conditional intestinal HuR deletion, studies in AIM 2 will examine the cell- specific role of HuR in post-transcriptional regulation of stem cell signaling pathways. We will examine enteroids from HuR-IKO, Apc Min-HuR IKO mice and from new lineage tracing models that will allow us to identify HuR-dependent targets in Lgr5+ progeny in order to study the impact on intestinal growth and tumorigenesis. Taken together, these studies will provide new insight into Apobec-1 dependent C-to-U RNA editing and modulation of both physiological and aberrant targets as well as expanded understanding of HuR dependent post-transcriptional regulation of Lgr5+ stem cell genes in intestinal growth and tumorigenesis.